Amplifying system



Jan. 13, 1942. A. H. TURNER 1 I AMPLIFYING SYSTEM Filed 001:. 22, 19:51

49 II II v 5 -22 52 4 48 35 lo n f ,==E 47 2 I N V EN TOR. Alfred H Tu rner.

KQAW w ATTORNEY.

Patented Jan. 13, 1942 UNITED STATES PATENT OFFICE AMPLIFYING SYSTEM Delaware Application October 22, 1931, Serial No. 570,318

13 Claims.

The present invention relates to amplifying systems and, more particularly, to superheterodyne systems for receiving alternating currents at radio frequencies.

An object of the invention is to provide a compound coupling system having a substantially constant amplification or transmission characteristic, for radio frequency circuits having a relatively wide frequency range of operation.

In accordance with the invention, one of the coupling elements may be provided at the high potential ends of two coupled circuits by the existing capacity in the tuning apparatus and at the low potential ends by a common, relatively large, capacity element, the coupling relation to the first capacity being in phase to transfer energy from one coupled circuit to the other, whereby a degree of coupling may be provided that varies in any desired manner with a change in frequency, or that may remain constant over a wide frequency range.

The invention will, however, be better understood from the following description when taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

vIn the drawing, Fig. 1 is a circuit diagram of a portion of a tuned radio frequency receiver;

Fig. 2 is a similar diagram of a portion of a superheterodyne receiver embodying my invention; and

Fig. 3 is a view in perspective and on a reduced scale of a variable tuning condenser arranged to provide a capacity coupling element for the circuit of Figs. 1 and 2.

Referring to Fig. l, a two stage radio frequency amplifier is shown, including two electric discharge devices or amplifier tubes 5 and 6, preferably of the screen grid indirectly heated cathode type. The amplifier is provided with input terminals 1 and output terminals 8. The tubes are supplied with suitable operating potentials through supply terminals indicated at 9 and bias and potential-adjusting resistors indicated generally at Ill and H in the cathode and screen grid circuits respectively. These circuits are also suitably by-passed by condensers indicated at I2.

The circuit shown and described may be taken as representative of any radio frequency amplifier having a pair of amplifier tubes adapted to be coupled in cascade for the transmission and amplification of radio frequency signals.

In the present example the amplifier tubes are coupled by a tuned coupling system indicated generally at l3 and including a tuned output transformer I4 for the first stage tube 5, and a tuned reactor 15 for the input circuit of the second stage tube 6. The shunt tuning capacitors for the transformer and for the reactor are indicated respectively at It and I! and are of the multiple type having a. common rotor l8 and separate stators l9 and 20, known as uni-control or gang condensers.

The condenser I 8 and its associated tuning inductance 2|, provide a tuned circuit 28 having a high potential end or lead 22 and a low potential connection lead 23. In a similar manner the condenser I1 and the inductance l5 provide a second tuned circuit 29 having a high potential end 24 and a common low potential connection with the lead 23.

The low potential lead 23 is connected to a common return lead 26 for the rotor element l8 of the variable condensers, through a capacity provided by a suitable fixed condenser 21. The condenser 2! is thus interposed in series with each of the circuits 28 and 29 in the low potential ends thereof and provides one element of a capacity coupling system between the circuits. A potential E1, existing in one circuit 28, across the coupling condenser 21, is thus applied in series with the other circuit 29, and in the present example to the input circuit of the tube 6. It will be noted that the low potential side of each circuit is provided with a common ground lead 25.

For low frequency coupling, the condenser 21 is of a relatively high capacity value. For example, in the broadcast band of frequencies, a value of substantially .005 microfarad has been found to be satisfactory. A fixed resistor, indicated at 30, may be connected in parallel with the condenser 21 to limit its effective circuit impedance in response to frequency changes, although in certain circuits this may be omitted. In the present circuit, the resistor 30 serves to connect the grid or input circuit to the cathode whereby the grid receives a proper bias potential.

The high frequency ends of the circuits 28 and 29 at any desired point, such as between the leads 22 and 24, are also capacitively coupled by a second fixed capacity provided by a condenser 3|. This is preferably of a very low capacity value, such for example as 2 micro-microfarads.

In a preferred embodiment of my invention this last capacity is also preferably provided by so relating the stator elements l9 and 20 of the tuning capacitors l6 and 11, that the desired capacity exists between them. For example, referring to Fig. 3, condensers l6 and I! are indicated in a preferred arrangement in which the stators l9 and are mounted in a common frame 32 in close, spaced relation as indicated at 32c, whereby the desired fixed capacity exists between them. It is obvious that in any type of variable tuning device, suitable arrangement may be made in a similar manner whereby the desired fixed capacity may exist between the electrodes or elements which are connected with the high potential ends of the circuits to be coupled.

It has been found that the above described coupling system, utilizing the two coupling capacitors 21 and 3|, has desirable frequency response characteristics such as substantially a uniform voltage transfer over the tuning range. In the circuit shown in Fig. 1 the capacities provided by condensers 2'! and 3| are in phase relation while at the same time they are in opposition in regard to their frequency characteristics.

In the above manner the capacity at 3| provides the major coupling between the circuits 2B and 29 at the high frequency end of the tuning range, while the condenser 21, which is substantially in the low frequency return lead 26 for both circuits, or common to both circuits to be coupled in the low frequency side thereof, provides the low frequency coupling between the two circuits 28 and 29.

In connection with tuned coupling circuits and in receiving apparatus having tuned radio frequency circuits, a fixed compound capacity coupling may thus be provided by so arranging the tuning condenser that only one fixed condenser need be employed, this being the low frequency coupling condenser, such as the condenser 21.

It will be appreciated that the capacities of the coupling condensers 21 and 3|, may suitably be varied to provide values for effecting a desired overall frequency response characteristic between the circuits 28 and 29 for the band pass coupling system It.

Referring to Fig. 2, together are indicated at 33 and 34, and include, respectively, an inductance 35 and variable tuning capacitor 36, and an inductance 31 and its corresponding variable tuningcapacitor 38.

two circuits to be coupled The inductance 31 is associated with an input same manner as the tube 6 in the circuit of Fig. '1.

The tuned circuit 34 forms the grid or input circuit of the detector 4| and is connected with the cathode through a lead 45 in which is connectedafixed coupling capacitor 46 and a shunt resistor 41, whereby the capacitor and resistor are included in the cathode returnlead from the tuned circuit.

Thecapacitor 46 and resistor 41 are also included in the tuned circuit 33, being interposed as indicated between the variable tuning capacitor 33 and the inductance 35 in the low potential side of the circuit. The .high potential ends of the circuits 33 and 34 are connected or coupled by a capacitor 52.

It will thus be seen thatsignals or .oscillatory currents existing in circuit 33 as from an oscillator as hereinafter described, may be applied to the grid of the detector tube through the coupling capacity of the condenser 52 and by the in-phase application of potentials existing across the capacitor 46 and the resistor 41 in the cathode return lead 45 of the detector tube 4|, whereby such potentials are applied between the cathode and the grid of the detector tube.

An oscillator tube 48 as shown in the drawing, may be connected with the inductance 35 and. the tuned circuit 33 of which it forms a part. The oscillator is provided with a suitable grid leak 39 and condenser 50, and a suitable feed back coil 5| associated with the inductance 35. With this arrangement it will be seen that the oscillations generated by the tube 48 and transmitted to the circuit 33 will be applied to the detector tube 3 The fixed capacitance coupling provided by the condenser 43 is effective at the low frequency end of the tuning range of the circuits 33 and 34 and is of a value suitable to provide the desired coupling. As mentioned in connection with the circuit of Fig. 1 this value may be substantially .005 microfarad for one condition of operation in connection with a broadcast receiver.

The high frequency coupling is provided by the condenser 52 connected between the high potential ends of the circuits 33 and 34 and may be of a value similar to that of condenser 3| of Fig. 2, whereby it is more effective at the high frequency end of the tuning range of the circuits 33 and 34. It has been found that a capacity of substantially 2 micro-microfarads in condenser 52 and a resistance of about 1500 ohms in the resistor 41, together with the above mentioned capacity for the condenser 45 is satisfactory for operation in a superheterodyne radio receiver.

It will be noted that the circuit shown in Fig. 2 is substantially the same as that of Fig. 1. The coupling capacitor 46 and the resistor are included in the circuit 33 at the low potential end thereof, after the manner of the coupling system shown and described in connection with Fig. 1. However, the circuit of Fig. 2 is modified to apply the low frequency coupling potential E2 in series in the cathode return lead of the detector tube 4|, instead of in series in the second tuned circuit, this being a preferred circuit arrangement for use in connection with a superheterodyne type of receiver, although it should be understood that the circuit of Fig. 1 may also be used. It will also be noted that in Fig. 2 the condenser rotors are directly grounded to a ground lead 53 similar to the circuit of Fig. 1.

It is obvious that, as shown and described in connection with Fig. l, the capacity provided'by the condenser 52 as a coupling means for the high frequency ends of the circuits 33 and 34 may be provided by the capacity existing between the proper elements of the condensers 36 and 38. Such elements may, for example, be the stators arranged substantially as shown and described heretofore in connection with Fig. 3.

The circuit arrangement of the system above described has the advantage that a fixed compound capacity coupling may thereby be provided between two circuits by simple means involving only two fixed condensers, one of which may be located between the high potential sides thereof and which may be established between the circuit elements, such as by the capacity existing between the stator elements of "the tuning capacitors, and the other being located in the low potential side of each circuit. Further- -more, by proper adjustment of the values of the two fixed capacitors independently of each other, any desired frequency response characteristic may be imparted to the coupling system. In the case that the capacity existing between the tuning capacitor stators is taken advantage of as a coupling means, it will be seen that only one additional fixed condenser need be employed.

The circuit has the further advantage that in connection with a superheterodyne type of receiver it may readily be adapted to apply the coupling potential at the low frequency end of the tuning range directly in the low potential cathode lead.

I claim as my invention:

1. In an electrical apparatus, a tuned oscillator circuit having tuning means interposed between high and low potential sides thereof, a fixed capacitor connected in series with said circuit in the low potential side thereof, an electric discharge device having a tuned input circuit including a high potential lead, and a cathode return lead, said input circuit and said cathode return lead being connected with said fixed capacitor whereby said capacitor is included in circuit between said tuned input circuit and said cathode return lead, and means providing fixed capacity coupling between the high potential sides of said circuits.

2. In a radio receiving system, th combination with an electric discharge device having a control electrode and a cathode, of a tuned input coupling means therefor comprising a pair of tuned circuits, capacity coupling means for said circuits including a coupling condenser in series in one of said circuits at the low potential end thereof, means providing a connection for the other of said tuned circuits with the control electrode and cathode of said device, said coupling condenser being included in said connection between the cathode and said other tuned circuit.

3. In a superheterodyne receiver, the combination with an electric discharge detector device having a control electrode and a cathode, of a tuned input circuit therefor including a tuning inductance, a coupling condenser interposed between said tuned input circuit and the cathode, and an oscillator circuit including a second tuning inductance, said last named circuit including said coupling condenser in series therewith.

4. In a superheterodyne receiver, the combination with an electric discharge detector device having a control electrode and a cathode, of a tuned input circuit therefor including a tuning inductance and a variable tuning condenser, a source of radio frequency potentials coupled to said circuit, a coupling condenser interposed between said tuned input circuit and the cathode, an oscillator circuit tuned to a frequency slightly different from that of said tuned input circuit including a second tuning inductance and a second variable tuning condenser, said last named circuit including said coupling condenser in series therein, means providing capacity coupling between said circuits at the high frequency ends thereof including the stator elements of said variable condensers, and common means for operating said rotors simultaneously for the variable tuning of said circuits.

5. In a superheterodyne receiving system, the combination with an electric discharge device having a control grid, a cathode and an anode,

of a tuned signal input coupling means therefor comprising a tuned circuit including a variable tuning capacitor and an inductance element, a series capacitor in said circuit, a second tuned circuit comprising a variable tuning capacitor and an inductance element, one terminal of each of said tuning capacitors and said last-named in-' ductance element being connected to ground and to said first-named inductance element through said series capacitor and means for connecting said cathode to ground through said series capacitor.

6. In a superheterodyne receiving system, the combination with an electric discharge device having a control grid, a cathode and an anode, of a tuned signal input coupling means therefor comprising a tuned circuit including a variable tuning capacitor and an inductance element, a series capacitor in said circuit, a second tuned circuit comprising a variable tuning capacitor and an inductance element, one terminal of each of said tuning capacitors and said last-named inductance element being connected to ground and to said first-named inductance element through said series capacitor, means for connecting said cathode to ground through said series capacitor, a second electric discharge devic coupled to said first-named inductance element and having a cathode connected to ground through said series capacitor and directly to the first-named cathode.

7. In a radio frequency signalling system; an electric discharge device; a source of radio frequency potentials; a compound coupling input circuit, tunable to select any frequency throughout a radio frequency tuning range, connected between said source and input electrodes of said device; said compound circuit comprising high frequency coupling means more effective in the transfer of potentials toward the higher end of said range, connected to one of said electrodes; said compound circuit comprising low frequency coupling means, more effective in the transfer of potentials toward the lower end of said range, connected to another of said electrodes.

8. The invention as set forth in claim 7, characterized in that said source of potentials is an oscillator in a superheterodyne receiving system and that said discharge device is a detector.

9. The invention as set forth in claim 7, characterized in that said high frequency and said low frequency coupling means ar adjustable to effect a desired overall frequency response characteristic in said input coupling circuit.

10. The invention as set forth in claim 7, characterized in that a second source of radio frequency potentials is connected to the input of said discharge device.

11. In a superheterodyne receiving system; an electric discharge device having a plurality of input electrodes; two sources of radio frequency potentials; a plurality of circuits simultaneously tunable to any frequency in a radio frequency tuning range; one of said circuits connected to one of said electrodes and coupled to one of said sources; a second of said circuits connected to a second of said electrodes and coupled to the second of said sources; said first of said circuits being coupled to one of said sources through high frequency coupling means more effective in the transfer of potentials toward the higher end of said range; the second of said circuits being coupled to said second electrode through low frequency coupling means more efiective in the transfer of potentials toward the lower end of said range.

12. In a superheterodyne receiving system, a thermionic detector tube having a plurality of input electrodes and a screen grid, a tuned circuit coupled to a source of radio signals and connected to one of said electrodes; said circuit being tuned by a variable capacitor to any frequency in a broadcast range; an oscillator comprising an inductance element shunt tuned by a variable capacitor in series with a fixed capacitor; means for coupling said oscillator to another of said electrodes, comprising said fixed capacitor.

13. In a superheterodyne receiving system, a detector tube having an indirectly heated cathode, a grid and an anode, an inductance element connected between said grid and ground and a capacitor connected between ground and said cathode, means for impressing a signal radio frequency potential upon said grid across said element, and means for impressing another radio frequency potential upon said cathode across said capacitor.

ALFRED H. TURNER. 

